1. Field of the Invention
The present invention relates to a brush device for an electric rotary machine such as a vehicular alternator.
2. Description of the Related Art
The brush device mounted on a vehicular alternator is mainly composed of brushes, brush copper wires, springs, and a brush holder. The brush holder is equipped with terminals made of metal and accommodates the brushes, the brush copper wires, and the springs therein. The brush copper wires are fixed to and electrically connected to the brushes by caulking and the like. The springs forcedly push the brushes toward slip rings that are mounted on a rotor shaft of a rotor. In the brush device, the brush copper wires are electrically connected to the terminals. U.S. Pat. No. 4,959,576 (corresponding to Japanese patent No. JP H7-32571) has disclosed such a brush device, for example.
Because the rotation of the rotor of the vehicular alternator causes wearing a contact part of each brush that contacts the surface of the corresponding slip ring, it is necessary to replace the worn brush with a new brush before reaching the lifetime of the worn brush. For example, it is necessary to dismantle or disassemble the vehicular alternator at a periodic inspection time in order to estimate the length of the worn part of each brush by measuring the magnitude of a spring force applied to the surface of the brush. However, because such a conventional manner requires dismantling or disassembling the brush device from the vehicular alternator, it takes a long inspection working time.
In order to avoid such a conventional drawback, namely to reduce the inspection working time, another conventional technique provides a brush device equipped with a brush having a concave part. The configuration of this brush device, an inspector can recognize a replacement time of the brush without dismantling the brush device from the vehicular alternator, namely without disassembling the vehicular alternator. Japanese patent laid open publication No. JP 2003-134745 has disclosed such a conventional technique. The concave part formed on the brush is hidden while the brush is not worn. In other words, because the concave part formed on the brush is not exposed on the surface thereof before a replacement time of the brush, the inspector can not observe the concave part from the outside of the vehicular alternator unless the brush device is dismantled. When the brush is worn off, the inspector can observe the concave part of the brush exposed to the outside for the first time, for example, he detects the exposed concave part at a following replacement time. According to the result of the inspection, the worn brush is replaced with a new brush.
In this conventional technique, the inspector can know the replacement time to replace the worn brush with new one by detecting the concave part exposed on the surface of the brush. Thus, this conventional technique only shows whether the concave part is exposed or not, that is, the inspector cannot know the detailed wear-condition of the brush in use. For example, if an abnormal wear-condition of the brush occurs by some reasons, which are advanced faster than a normal use, the inspector cannot recognize the occurrence of the abnormal wear-condition unless the concave part of the brush is exposed to the outside.
In addition, because there is a possibility of exposing the concave part on the brush immediately following the periodic inspection time, it is necessary to form the concave part at the optimum position in order to continuously use the brush for a short period of time after the exposure of the concave part on the brush. However, there is a possibility of replacing a worn brush with a new one if the position of the concave part is separated from the optimum position. This case is not economical. On the contrary, even if the position of the concave part is formed at a position almost equal to a optimal serviceability limit position, there still remains a possibility of occurring the break of the brush before the following inspection time. This introduces difficulty in continuous use of the brush in safe. In particular, because the travel distance fluctuates from vehicle to vehicle even if the periodic inspection is carried out at a regular period of time, it is in general difficult to satisfy both the economy and safety simultaneously through all vehicles, in view of wear of the brush in the brush device, for example.